(a) Field of the Invention
This invention pertains generally to encoding of binary signals, and in particular, to a technique of encoding a sequence of bits for transmission on a quadrature amplitude modulated (QAM) carrier signal, in which convolutional encoding and mapping of the encoder output to an expanded set of signalling alphabets are used to reduce the effect of channel impairments without sacrificing data rate or requiring more bandwidth, and in which differential encoding is used to remove the phase ambiguity of the expanded set of signalling alphabets.
(b) Description of the Prior Art
U.S. Pat. No. 4,077,021 issued to I. P. Csajka and G. Ungerboeck on Feb. 18, 1978, which is incorporated herein by reference, describes a technique for converting a sequence of binary digits into a sequence of signalling alphabets of a modulated carrier signal for data transmission. The invention is intended to allow recovery of the original data even in situations where the transmission medium is severly impaired. Generally speaking, the Ungerboeck's invention involves applying groups of r input bits to a finite state machine which is arranged to expand each input group into an r+1 bit group in accordance with predetermined logical combinations with certain bits in previous groups. The number P of bits stored in the encoder that are used to form the encoder output determine the number m(=2.sup.p) of states that the encoder may assume. Transitions from each encoder state to other states must follow prescribed rules. Accordingly, when the output of a convolutional encoder is subsequently used to modulate a carrier signal in accordance with in-phase and quadrature phase coordinates obtained by mapping the output of the convolutional encoder to an "expanded" set of 2.sup.r+1 signalling alphabets (sometimes referred to as a signal constellation), the sequence of signalling alphabets must follow prescribed rules. (The constellation is referred to as "expanded" because, conventionally, in order to transmit r bits in a signalling interval, a signal constellation of 2.sup.r signalling alphabets would suffice.) At the receiver, the effect of impairments in the transmission medium which would otherwise impede data recovery are largely overcome by a maximum-likelihood decoding algorithm which determines the correct transmitted data using knowledge of the valid sequences of signalling alphabets. A discussion of one such decoding algorithm is contained in a paper by G. D. Forney Jr. entitled "The Viterbi Algorithm", Proc. of IEEE, Vol. 61, No. 3, March 1973, pp. 268-78.
Despite the advantages obtained by use of the encoding technique described by Csajka and Ungerboeck, phase hits or jumps occurring in the transmission medium may result, after recovery of equalization and carrier, in a rotation of the received signalling alphabets as compared to the initial determination of phase. This ambiguity in phase can cause errors in all subsequently received data and thereby seriously degrade the performance of the system. To avoid this problem, it would be desirable to apply a differential encoding technique to the original input data so that the received signalling alphabets, even after a rotation, can be used to recover the original data. However, to date, it has not been thought possible to combine, in a single system, the advantages of encoding as taught by Csajka Ungerboeck with a differential encoding technique.
In view of the foregoing, it is the broad object of the present invention to provide a technique and apparatus for converting a sequence of binary digits into a sequence of signalling alphabets of a modulated carrier for transmission on a medium subject to impairments such as phase hits and phase jumps, so that the original data can be accurately recovered. Specifically, it is desired to combine the advantages obtained by Csajka encoding as taught by Ungerboeck with those associated with differential encoding.